This invention relates generally to electrical transmission devices and particularly to the bushings provided therein for passing a current-bearing conductor through the metal enclosure surrounding those devices which operate within a reservoir of dielectric fluid.
In electrical power distribution systems, many components such as distribution transformers are located at points remote from other system components with interconnections being supplied by networks of transmission lines. Because such system components are used in the distribution of electrical power to consumers, they are expected to handle large operating currents. Distribution transformers, for example, are located at various points within the power distribution network and provide a change of operating voltage from the higher potential main supply to a lower potential consumer supply. Generally, power line distribution transformers are situated within metal housings or enclosures and are immersed within a dielectric fluid such as oil. The oil provides both cooling of the transformer windings and core as well as increased electrical insulation and protection from moisture. The transformer windings are electrically connected to the remainder of the system external to the enclosure by conductors passing through apertures in the metal enclosure. The conductors passing through the enclosure are, of course, electrically insulated from the enclosure by interposed bushing structures which take a number of forms in the art, such as porcelain bushings. Unfortunately this electrical isolation usually produces thermal isolation, such that the dielectric bushing or insulator reduces the ability of the conductor to dissipate the heat generated by the conduction of electrical current.
Because the reliability of both insulators and electrical devices may be reduced by excessive heating, practitioners in the art have endeavored to reduce the operating temperature of such bushing enclosed conductors by providing various cooling means. One rather straight-forward solution envolves simply enlarging the conductor size, thereby providing a greater heat capacity, lower resistance, and greater heat dissipating area. A more effective heat dissipation system is provided by structures which immerse a portion of the conductor in the cooling dielectric fluid of the transformer. A still better system of heat dissipation is provided by using circulation of the dielectric fluid around the current-bearing conductor by either a forced flow or convection. The structures utilizing convective flow for cooling rather than forced flow have particular advantage in power distribution transformers or other remotely located equipment because it is usually difficult to provide a reliable source of fluid pressure.
While structures embodying one or more of the preceding improvements have provided enhanced current carrying capability, some improvement is still desired. One problem of previous systems arises because the heat produced by a conductor bearing a large current is, of course, caused by resistance within the conduction path. In structures of the type used to interconnect the internal transformer portions to the remainder of the system, the total conductive path typically includes a group of several separate conductive parts fastened together by mechanical fasteners such as threaded combinations. Because of any number of variables arising during the manufacturing of components, such as tolerances and inconsistencies in plating and finishing, a high resistance may exist in the assembled connector which is somewhat localized. When this connector is subjected to a substantial electrical current, the localized resistance produces a "hot spot"; that is, a portion of the connector becomes heated substantially more than the remainder of the connector. Because the exact location of such hot spots in assembled connectors is in many cases random, it is desirable to provide a cooling system for the connector in which the flow pattern of the dielectric fluid can increase in the area of such hot spots.
Accordingly, it is an object of the present invention to provide an improved connector bushing for use in a transformer enclosure of the like which makes optimum use of the supply of cooling dielectric fluid. It is more specific object of the present invention to provide an improved bushing in which the flow of cooling fluid varies in response to temperature differences between portions of the connector bushing.